Semiconductor optical amplifiers (SOAs) are promising as small-size, low-cost optical amplifiers to replace erbium-doped fiber amplifiers (EDFA) that are typically used in conventional optical communication systems.
One of the goals to be achieved with respect to the performance of SOAs is to increase the saturated output. Generally, the maximum optical output of amplified light generated by SOAs is lower than that of EDFAs. At present, SOAs, which are considered to be more advantageous than EDFAs for their smaller sizes and lower costs, have not been available as a substitute for EDFAs. For SOAs to replace EDFAs in large-capacity wavelength-multiplex optical communication systems, the SOAs need to produce a saturated output of at least 20 dBm (100 mW). In efforts to increase the saturated output, care should be taken not to sacrifice general performance details required by optical amplifiers and performance advantages of SOAs, including polarization dependency, drive current, device size, etc.
A general process for improving the saturated output of SOAs is to optimize the structure of an active layer while maintaining the propagation characteristics of a single transverse mode which is indispensable for optical communication devices. A first conventional arrangement of this process, represented by an SOA by K. Morito, et al. (see non-patent document 1: European conference of optical communications'2000 (ECOC'2000) International Meeting, paper 1.3.2), will be described below.
According to the first conventional arrangement, a tensile-strained bulk active layer which can more easily be rendered independent of polarization than a multiple-quantum well (MQW) that is widely used as an active layer in semiconductor lasers is employed, and the thickness of the active layer is reduced to about 50 nm thereby to reduce a light confinement coefficient for confining light in the active layer to about 0.1, so that the internal loss of the active layer is reduced and a waveguide mode cross-sectional area (a light confinement coefficient per unit active layer area) is reduced. This structure makes it possible to achieve such high characteristics as a fiber-to-fiber gain of 19 dB and a fiber saturated output (which is defined as a fiber optical output intensity in the case where the fiber-to-fiber gain is 3 dB lower than the gain G0 when not saturated) of 17.4 dBm under such drive conditions as an injected current of 500 mA and an injected current density of about 30 kA/cm2.
However, it is highly difficult at present to achieve a saturated output of at least 20 dBm through the optimization of the active layer according to the first conventional arrangement, for the following reasons:
1) It is difficult to achieve both a high saturated output and polarization independence.
2) The drive current density is increased.
3) The gain saturation characteristics with respect to a signal light are deteriorated.
The above three factors will be described below in specific detail. With regard to the factor 1), it is necessary to further reduce the waveguide cross-sectional area in order to improve the saturated output. If, however, the waveguide cross-sectional area is to be further reduced to obtain a saturated output of 20 dBm in the first conventional arrangement, the thickness of the active layer needs to be reduced to about 20 nm or less. If the thickness of the active layer is reduced, a crystal strain required for polarization independence will be increased, resulting in crystal growth difficulty. The thin active layer develops a quantum effect, causing the active layer to have characteristics as a multiple-quantum well which is largely polarization-dependent. Therefore, it is difficult to achieve polarization-independent gain characteristics.
With respect to the factor 2), an injected current density in excess of 30 kA/cm2 reported in the first conventional arrangement is required in order to obtain a saturated output of 20 dBm. However, using such a high current density as a drive condition is not practical in view of deteriorated gain characteristics due to heating and device reliability.
With respect to the factor 3), the saturated output performance on dynamic characteristics for amplifying a modulated signal is not yet sufficient. This is a problem caused by the definition of a saturated output which serves as an index for estimating the maximum optical output of an SOA. As described above, the saturated output of an optical amplifier has heretofore been defined generally as a fiber optical output when a gain reduction from an unsaturated gain is 3 dB, and represented by “P3 dB”. EDFAs that are typically used in conventional optical communication systems do not cause a signal waveform deterioration, but cause only a gain reduction, when amplifying a signal in the gain saturation range. SOAs, however, cause a signal waveform deterioration when driven in an optical output intensity range with a gain reduction of 3 dB. Such a signal waveform deterioration is due to the essential nature of SOAs that they are susceptible to a gain saturation relaxation process because the carrier relaxation time in the active layer and the bit rate of the signal light are of about the same order of time.
The above waveform deterioration is also reported in the non-patent document 1 cited as representing the first conventional arrangement. The non-patent document 1 illustrates amplified signal waveforms of a signal having a bit rate of 10 Gb/s respectively for optical outputs of 12 dBm and 14 dBm according to the results of an experiment on transmitted signal waveforms at the time a modulated signal is applied. It is reported that a signal waveform deterioration is confirmed when the SOA is driven to produce an optical output of +14 dBm and a slight signal waveform deterioration is observed when the SOA is driven to produce an optical output of +12 dBm.
In view of the above waveform deterioration peculiar to the SOAs, an optical output when a gain reduction from an unsaturated gain is 1 dB is newly defined as a saturated output which does not cause a signal deterioration, and is represented by “P1 dB” P1 dB in the conventional arrangement is at most about 10 dBm, indicating that a further increase in the saturated output is required.
As described above, it can bee seen that it is highly difficult to achieve a maximum optical output in excess of 20 dBm with the active layer according to the process of optimizing active layer while maintaining the single transverse mode.
There has been proposed a process of solving the 1) and 2) of the above three limitations. According to the proposed process, the input optical intensity per unit active layer area is lowered by increasing the effective width of the active layer while maintaining the optical output in the single transverse mode, using a tapered waveguide or an interference waveguide. Conventional arrangements based on the proposed process will be introduced below.
First, “multi waveguide (MWG)-SOA” disclosed by B. Dagens, et al. (see non-patent document 2: IEEE Electronics Letters, vol. 35, No. 6, pp. 485-487, 1999) is illustrated as a second conventional arrangement. An arrangement and operating principles of an MWG-SOA according to the second conventional arrangement will be described below with reference to FIGS. 1 and 2. As schematically shown in FIG. 1, the MWG-SOA comprises an array of N active layers 200 and passive waveguides 201 disposed at opposite ends of N active layers 200. Each of passive waveguides 201 includes 1×N optical multiplexer/demultiplexer 202. Input light (having intensity Pin) is demultiplexed by optical multiplexer/demultiplexer 202 at the front ends of active layers 200 into N input lights which are applied to respective active layers 200. Each of the input lights which are applied to respective active layers 200 has optical intensity Pin/N. The lights are amplified by respective active layers 200 and then multiplexed by optical multiplexer/demultiplexer 202 at the rear ends of active layers 200 into output light which is emitted. The N optical paths for the respective lights have the same length. Since the N lights propagated through the respective optical paths are multiplexed in phase with each other by rear optical multiplexer/demultiplexer 202, no demultiplexing loss is developed.
FIG. 2 is a graph showing gain saturation characteristics of the device according to the second conventional arrangement. The graph has a horizontal axis representing the intensity of the optical output from the SOA and a vertical axis the device gain, and shows gain saturation characteristics plotted when the number of active layers of the array is 1, 2, and 4. The gain saturation characteristics for N=1 correspond to the characteristics of a conventional SOA in the signal transverse mode. As described above, the gain characteristics of the SOA are such that the gain is a constant unsaturated gain G0 when the input optical intensity is small, but as the input optical intensity increases, the gain drops and the optical output intensity is saturated. In the arrangement shown in FIG. 2, the newly defined optical output “P1 dB” is not employed, but the conventional optical output “P3 dB” is employed. The saturated output for N=1 is indicated by “a” in FIG. 2. When N=2, since the input optical intensity per active layer becomes half, the saturated output of the entire device is indicated by “b” in FIG. 2, and improved twice (=3 dB) the output for N=1. Similarly, when N=4, since the input optical intensity per active layer becomes ¼, the saturated output of the entire device is indicated by “c” in FIG. 2, and improved four times (=6 dB) the output for N=1.
According to the non-patent document 2, it is reported that the saturated output of the SOA in the single mode was improved 5 dB. Though the drive current flowing to the array of N active layers is N times the drive current for N=1, the injected current density for the array of N active layers remains the same as the injected current density for N=1. Therefore, it will be understood that the injected current density is not increased with respect to the difficulty 1) to be eliminated for increasing the saturated output. According to this process, since it is not necessary to reduce the mode cross-sectional area of the active layer, the thickness of the active layer may be of 50 nm which is about the same as the thickness according to the conventional arrangement. Therefore, it will be seen that the problems of the factor 2) referred to above is solved.
Another report on an increased waveguide width for an increased saturated output is shown in “active-MMI (Multimode-interference)—SOA” disclosed by K. Hamamoto, et al. (see non-patent document 3: IEEE Electronics Letters, vol. 36, No. 14, pp. 1218-1220, 2000), which is illustrated as a third conventional arrangement. A planar structure of the device is schematically shown in FIG. 3.
As shown in FIG. 3, the MMI-SOA has single-mode waveguide 101 disposed on device end face 103 to which input light is applied, single-mode waveguide 102 disposed on device end face 104 from which output light is emitted, and multimode interference (MMI) waveguide 100 disposed between these waveguides 101, 102. Each of single-mode waveguides 101, 102 has width W1, and MMI waveguide 100 has width W2 (>W1). The width and length of MMI waveguide 100 are designed such that MMI waveguide 100 functions as a 1×1 multiplexer/demultiplexer based on the propagation principles of the MMI waveguide. Light that is applied from the input-side single-mode waveguide and propagated through MMI waveguide 100 can be taken out again as transverse single-mode light without a demultiplexing loss.
MMI waveguide 100 can reduce the input light density because of a wide active layer thereof, as with the MWG-SOA according to the second conventional arrangement, and is capable of realizing an SOA with a high saturated output. According to the non-patent document 3, it is reported that a saturated output improvement of 5 dB was achieved by an SOA capable of producing a single transverse mode output.
One promising solution to the problem according to the factor 3) to be eliminated for increasing the saturated output is a gain-clamped SOA. The gain-clamped SOA is an SOA incorporating a laser resonator structure therein for amplifying signal light that is applied to a laser-oscillated active layer. The gain-clamped SOA is known as a technology for improving a signal waveform deterioration due to an SOA gain saturation and interchannel crosstalk that is developed when a wavelength-multiplex signal is amplified, to achieve linear gain characteristics which do not depend on the input light intensity. Operation of the gain-clamped SOA will be described below.
FIG. 4 is a diagram illustrative of the gain saturation characteristics of a conventional SOA and a gain-clamped SOA. It is assumed that the conventional SOA and the gain-clamped SOA have the same active layer quality and the same drive current. As already described above with respect to the first and second conventional arrangements, when the SOA drive condition is constant, when the output light intensity is increased, the gain of the SOA is saturated and becomes lower than the unsaturated gain G0.
The gain saturation curve of the conventional SOA is indicated by the broken line in FIG. 4. The saturated output P1 dB of the SOA is indicated by “a” in FIG. 4. The gain saturation curve of the gain-clamped SOA is indicated by the solid line in FIG. 4. As described above, the gain-clamped SOA has a laser oscillator structure incorporated therein and is actuated in a laser-oscillated state. In the laser-oscillated state, the carrier density in the active layer of the SOA is clamped to the oscillation threshold carrier density of the laser. At this time, the gain of the gain-clamped SOA does not depend on the output light intensity, but is constant as an unsaturated gain GGC. The unsaturated gain GGC can be set to a desired level by changing the reflectance of the laser oscillator incorporated in the gain-clamped SOA. According to the gain-clamped SOA, the gain is also saturated when the output light intensity increases. In the gain saturation range, since the number of carriers consumed for amplifying the input light increases, the laser-oscillated state cannot be maintained, shutting off the gain clamping operation. The output light intensity (“b” in FIG. 4) at this time represents a saturated output value Gsat of the gain-clamped SOA. Though the saturated output value Gsat of the gain-clamped SOA is lower than the unsaturated gain G0 of the conventional SOA with the same quality under the same drive condition, it is possible to obtain a desired gain by designing the active layer in anticipation of such a gain drop.
As described above, the saturated output of the active layer having the same quality is higher with the gain-clamped SOA by “b-a (dB)” than with the conventional SOA, and it will be seen that the limitation of the factor 3) to be eliminated for increasing the saturated output is improved.
A reported arrangement of the gain-clamped SOA is shown in “gain-clamped (GC)—SOA” disclosed by M. Bachmann, et al. (see non-patent document 4: IEEE Electronics Letters, vol. 32, No. 22, pp. 2076-2077, 1996) (fourth conventional arrangement). FIG. 5 schematically shows a device structure of the GC-SOA. The fourth conventional arrangement has a waveguide structure including a resonator for oscillating clamped light, the resonator being disposed in the same optical path in a single-stripe active layer.
As shown in FIG. 5, the GC-SOA has a waveguide structure including distributed Bragg reflectors (DBRs) 111, 112 disposed respectively in the opposite ends of active layer 110 of an SOA. When the SOA is driven by a current injected into active layer 110, a laser oscillation occurs between DBRs 111, 112 at a certain wavelength (e.g., 1.5 μm) depending on their reflection peak, and laser-oscillated light (hereinafter referred to as clamped light) is emitted from entrance and exit ends of the waveguide structure. At this time, since active layer 110 is clamped to the oscillation threshold gain of the clamped light, it can give a constant gain at all times to the input light regardless of the intensity of the input light. This structure requires a filter on the exit end for removing the clamped light because the signal light and the clamped light travel through the same optical path.
Another reported arrangement of the gain-clamped SOA is an SOA (fifth conventional arrangement) disclosed in patent document 1 (Japanese laid-open patent publication No. 2000-77771). FIG. 6 schematically shows a device structure of the fifth conventional arrangement. The fifth conventional arrangement employs a Mach-Zehnder-interferometer waveguide structure for spatially separating signal light and clamped light at entrance and exit ends though the signal light and the clamped light pass through the same active layer. The active layer according to the fifth conventional arrangement corresponds to the structure wherein the number N of arrayed active layers of the SOA according to the second conventional arrangement is 2.
As shown in FIG. 6, the SOA has two ports A, B at one end and two ports C, D at the other end. Port A is an input port to which input light guided through convergent optical fiber 121 is applied. Port D is an output port for emitting output light which is guided outwardly by convergent optical fiber 122. Port B is combined with reflector 123, and port C is combined with reflector 124 and variable light attenuator 125. Ports A, B are connected to respective two input waveguides of optical multiplexer/demultiplexer 126 serving as an optical power equal distributor. Optical multiplexer/demultiplexer 126 has two output waveguides connected to respective two input waveguides of optical multiplexer/demultiplexer 127. These waveguides serve as respective interference arms of a Mach-Zehnder interferometer. Optical multiplexer/demultiplexer 127 has two output waveguides connected to respective ports C, D. The interference arms of the Mach-Zehnder interferometer have SOA units 120a, 120b formed respectively therein.
With the above SOA, input light supplied from port A is equally distributed by optical multiplexer/demultiplexer 126. Lights that are equally distributed by optical multiplexer/demultiplexer 126 are amplified by SOA units 120a, 120b in the interference arms, and then interference-multiplexed by optical multiplexer/demultiplexer 127 into amplified light, which is emitted from port D. When the SOA is driven, a laser oscillation occurs in a path interconnecting ports B, C between reflectors 123, 124, and clamped light is emitted from reflectors 123, 124. In this case, the gain is clamped in the same manner as shown in FIG. 2. Since the path of the signal light (the path interconnecting ports A, D), and the path of the clamped light (the path interconnecting ports B, C) are spatially separate from each other, a filter for removing the clamped light is not required. Therefore, it is expected that the cost of the module may be reduced.
With the structure shown in FIG. 6, the effective reflectance of reflector 124 can be adjusted by variable light attenuator 125 (or variable optical amplifier) disposed in a portion of the path of the clamped light. The structure shown in FIG. 6 is also advantageous in that the clamped light and the signal line may have the same wavelength.
Still another reported arrangement of the gain-clamped SOA is shown in “linear optical amplifier (LOA)” disclosed by D. A. Francis, et al. (see non-patent document 5: Technical digest of Optical Fiber communications 2001 (OFC2001), post deadline paper PD13). FIG. 7 schematically shows a device structure of the LOA. The LOA includes resonators disposed in vertically sandwiching relation to a waveguide through which signal light is propagated.
As shown in FIG. 7, active layer 131 having a waveguide structure is disposed in a substrate surface of InP substrate 130, and InP cladding 132 covering active layer 131. InP substrate 130 on which active layer 131 and InP cladding 132 are disposed is vertically sandwiched by a pair of DBRs 133, 134. The LOA is of a vertical cavity surface emitting laser (VCSEL) structure which is laser-oscillated in a direction perpendicular to the substrate surface.
With the above LOA, signal light is guided through waveguide-structure active layer 131 disposed in the substrate surface of InP substrate 130. DBRs 133, 134 disposed above and below active layer 131 cause a laser oscillation in the direction perpendicular to the substrate surface of InP substrate 130, emitting clamped light upwardly and downwardly of InP substrate 130. Though the LOA is of a single-stripe structure, it has separate optical paths of the signal light and the clamped light. Therefore, as with the SOA shown in FIG. 6, the LOA does not need a filter for removing the clamped light.
The features of the conventional designs have briefly been described above. Difficulties to be eliminated by the present invention are summarized as follows:
As described above, it is highly difficult to achieve a saturated output of at least 20 dBm mainly through the optimization of the active layer for the following reasons: 1) It is difficult to achieve both a high saturated output and polarization independence. 2) The drive current density is increased. 3) The gain saturation characteristics with respect to a signal light are deteriorated.
The process a) of improving the difficulties 1), 2) by increasing the effective waveguide width using an interference waveguide, and the process b) of improving the difficulty 3) with the gain-clamped SOA have been described above as conventional techniques. For achieving a saturated output in excess of 20 dBm in view of the required drive current, the above improvements a), b) still have problems to be solved. The reasons for those problems will be described below.
First, the problems of the process a) of improving the difficulties 1), 2) by increasing the effective waveguide width while maintaining a single transverse mode output will be described below. As described above with respect to the conventional arrangements, if the effective waveguide width can be increased N times the width W of the active layer for conventional single transverse mode propagation according to the present process, then the saturated output is improved N times. At this time, the injected current density remains the same. To achieve these characteristics, however, a drive current which is N times the drive current in the conventional SOA is required. A drive current of an SOA having a waveguide width of N×W when the saturate output P1 dB is 20 dBm is estimated to be about 2 A. This drive current is higher than the drive current of an LD for exciting an EDFA. It can be seen that because of the drive current, the present process is not sufficient for realizing an SOA which can be used as a substitute for an EDFA.
The process b) of improving the difficulty 3) with the gain-clamped SOA will be described below. As described above with respect to the conventional arrangement, the gain-clamped SOA can produce a higher saturated output than the conventional SOA even if they have active layers of the same quality. Therefore, if the same target saturated output value is to be achieved, then the gain-clamped SOA has a smaller drive current than the conventional SOA. However, inasmuch as the gain saturation curve itself is not improved by employing the gain-clamped SOA, the saturated output is improved by a value within the range of the gain saturation curve of the active layer which is not associated with gain clamping.
It is an object of the present invention to provide an SOA which has a high saturated output in excess of 20 dBm that has been difficult to achieve with the conventional SOA and which has such practical performance that it can be used as a substitute for an EDFA.